1. Field of the Invention
The invention relates in general to the control of high-powered lasers, and, in particular, to improving the reliability, safety and power output of free electron lasers.
2. Description of the Prior Art
Manmade space satellites are limited by their power requirements. Solar panels have long been used to provide power for satellites. Solar panels are limited for two reasons. First the energy received from the sun is relatively diffuse and is not adjustable. For a fixed surface area the power that can be created by the solar panel is limited. The greater the power, the larger the surface area required. Mirrors have been used in space to reflect more sunlight on a fixed area. This leads to the second limitation, the mass and volume of the solar panel needed to generate the required power level. For high power levels the mass and volume can force reductions in the remaining portions of the satellite. Even the use of mirrors reduces the mass and volume available for launch payloads. The power limitations force most satellites to be designed to operate on the power requirements of a small household appliance.
Electrical power has always been a limiting factor for satellites, and it restricts the services that they can perform. The need for additional transponders to satisfy the demand for satellite-supplied television, e-mail, worldwide web, long distance telephones, rapid computer data transfer, and many other types of telecommunication is increasing. The number of transponders has risen from about 24 active transponders per satellite in the late 1980""s to 94 active transponders on the Hughes satellite launched in late 1997. The demand for additional power over the last few years fits an exponential curve and the end is not in sight. Instead it is increasing even faster. In response to the need for additional frequencies to carry the load the Federal Communications Commission has opened up the Ka band for satellite use. Recently completed National Aeronautics and Space Administration (NASA) studies indicate that the Ka band will on occasion require the availability of up to ten times the power requirements of the lower frequency L, C and Ku bands to counter rain fade.
The additional power needed for the Ka band is required to keep the television signals broadcast to earth at an acceptable level of quality. The Ka frequency band is 26.5 to 40 gigahertz (wavelength range of 11 millimeters to 7 millimeters). The size of raindrops is typically 1 to 3 millimeters with a maximum of 5 to 7 millimeters. Although the Ka band is in a water absorption region, much of the extinction of the signal is caused by the resonance in scattered energy resulting from the proximity of the wavelength to the raindrop size. The only practical source for the additional power required is additional electricity from the solar panels carried on the satellite.
At present the size of satellite solar panels is awkwardly large. Additional satellites in the same xe2x80x9cspace slotxe2x80x9d can be deployed to increase the total solar panel area, and this is the direction that many satellite companies are going. A major drawback of this approach is that the output signals of the various satellites are not in phase, so interference between satellite transmissions can be a problem. The biggest drawback, however, is that the multiple satellite approach is very expensive.
One way to repower the satellites is laser power beaming, LPB. Laser beams can increase the power level an order of magnitude above that available from the sun. Further, since a laser can be tuned to a narrow frequency range, the efficiency of the solar panels can be maximized by using their optimum generating wavelength. Beaming from the earth""s surface requires the laser beam to travel through our planet""s atmosphere. For LPB to be effective the atmospheric path must be free of clouds. In addition, the atmosphere can cause various other problems associated with turbulence, scatter, and absorption. Sites with clear, smog-free air minimize the last two problem areas. Turbulence is handled by the use of adaptive optics.
Free electron lasers, FELs, are capable of high power without significant wavefront distortion since the light is generated in a vacuum. An ignition feedback regenerative free electron laser (FEL) amplifier (IFRA FEL) designed by Kim, Zholents and Zolotrev does not need cavity mirrors, a power-limiting feature of most types of lasers. It provides greater output power than prior FELs. By feeding a portion of the output power back through the undulator and a portion to the photocathode emitting electrons, optical power grows without the necessity of cavities. Their invention is disclosed in U.S. pat. application Ser. No. 09/361,675. Other prior art patents for free electron lasers include Sheffield U.S. Pat. No. 5,960,013 and Edighoffer U.S. Pat. No. 5,029,172. One problem with IFRA FEL is that the portion of the beam fed back can grow in power, increasing the output of the laser and going into a runaway situation. A further problem with IFRA FEL is that the generated optical pulse is of the same duration as the electron bunch that emitted the pulse. This causes phasing problems unless the optical feedback loop can be accurately controlled. Any portion of an electron bunch out of phase with the optical pulse is wasted. This invention provides a means for phasing of the feedback loop and a means for control of the internal efficiency within the undulator so positive feedback producing a runaway condition does not occur. This invention provides a method and mechanism to lengthen the duration of the optical pulse fed back to the IFRA FEL.
Limiter optics for an ignition feedback regenerative free electron laser (FEL) amplifier, IFRA FEL, are made by using a very small pickoff mirror to direct a portion of the IFRA FEL""s output beam to a pulse expander mirror. Light reflected from the expander mirror is then passed through focusing optics, which refocuses this pulse expanded light beam to a predetermined location within the undulator of the IFRA. FEL. Since it lengthens the duration of the IFRA FEL laser pulse, the next electron bunch is assured of being completely illuminated by the returned optical pulse.
The focusing optics has two Cassegrainian mirror systems. The first Cassegrainian mirror focuses the parallel light beam incident on a convex pickoff mirror to a focal point. The second Cassegrainian mirror then refocuses the light beam into a nearly parallel beam to a focal point in a predetermined location in the undulator. Mounted near the focal point between the two Cassegrainian mirrors is a movable limiter plate which controls the cutoff intensity of the pickoff beam.
In order to meet the phasing conditions, the optical pulse-length exiting the undulator must be expanded. In this way the optical pulse is not required to exactly match the electron bunch length time-wise. The feedback loop average power is planned in the IFRA FEL to be 4 kilowatt. A cooled mirror can handle over 2 kilowatts per square millimeter of surface area.